High reactivity, uncooperativeness, and lack of impulse control are characteristics of behavior problems in early childhood. The inability to control or regulate emotions, particularly negatiVe emotions, may be an important antecedent of these disorders. The goal of the present study is to identify these antecedents in a normal population by examining the development of emotion regulation during the first two years of life. Since the ability to modulate emotional arousal is likely the product of both the infant and the infant's social context, this study will also examine the physiological and social correlates of emotion regulation. Specifically, a longitudinal study of infants and their mothers is proposed. Infant emotional reactivity to a naturally occurring stressor (inoculation) and less aversive laboratory procedures (frustration tasks) and the infant's attempts to reduce or control his/her reactivity will be observed. Maternal responses to her infant's distress/frustration will also be assessed during these procedures, as well as during an early (two weeks) feeding interaction and a free play at 6 and 12 months of age. To assess early temperamental style over an extended period, mothers will keep a 5-day cry diary at two weeks and two months. Physiological correlates of reactivity and regulation will be obtained during all laboratory procedures. Finally, at 24 months of age subjects will participate in several compliance tasks and both parents will complete a questionnaire about their child's behavior.